01Absorbance tracks concentration
A spectrophotometer measures how much light a solution absorbs. A = εbc: absorbance is proportional to concentration (c), path length (b), and molar absorptivity (ε).
Choose the wavelength of maximum absorbance. Make a calibration line from standards and read unknowns off it.
02Notebook box
Doubling c doubles A. Fingerprints or a dirty cuvette raise A → concentration reported too high.
Worked example: Standard 0.10 M gives A = 0.40. Unknown A = 0.20 → 0.050 M.